As an apparatus which forms a thin film (e.g., silicon nitride (Si—N) film) on a substrate such as a semiconductor wafer (hereinafter, referred to as a “wafer”), there is known a batch type vertical heat treatment apparatus which forms a film on each of the wafers received in a vertical reaction tube at a time. A specific film forming method using such an apparatus includes a so-called ALD (Atomic Layer Deposition) technique that alternately supplies, a plurality of times, for example, a silicon-containing source gas and a reaction gas (e.g., ammonia (NH3) gas) for nitriding the source gas.
An object to which the silicon nitride film is applied may include a sidewall for protecting a lateral side of a gate electrode, among for example, a variety of parts constituting a semiconductor device. Specifically, a substantially cylindrical layered structure of a gate insulation film followed by a gate electrode is formed on a surface of a wafer. A silicon nitride film is formed to cover the structure. Thereafter, a mask is formed leaving only the silicon nitride film on a lateral surface of the structure. Subsequently, the silicon nitride film is subjected to a wet etching by, e.g., an etchant such as a hydrofluoric acid (HF) solution, thus forming the sidewall. Subsequently, contact electrodes for wiring between the gate electrode and drain and source regions are formed by performing, e.g., a sequence of processes of forming an insulation film such as a silicon oxide film; forming through-holes in the insulation film; and filling the through-holes with a metal material. The drain and source regions are formed in the vicinity of the gate electrode on the surface of the wafer.
In recent years, with the miniaturization of a wiring structure, the area between the gate electrode and the contact electrodes tends to be narrowed. A parasitic capacitance also tends to occur between the gate electrode and the contact electrodes. Therefore, to suppress an electrical coupling between the gate electrode and the contact electrodes, a dielectric constant (k value) of the sidewall should be properly lowered. As an example, doping the silicon nitride film with oxygen (O) is under consideration.
However, when the silicon nitride film is doped with oxygen, the sidewall is prone to be etched in the wet etching process. As such, in a case where the silicon nitride film is applied as the sidewall, doping the silicon nitride film with both oxygen and carbon (C) is under consideration as way to obtain an etching resistance of the sidewall.
As described above, the doping of the silicon nitride film with carbon generates a composition where a portion of the silicon element in a basic skeleton structure (Si—N) is replaced with the carbon element, which fails to obtain a thin film having a desired composition ratio. In a film forming cycle of alternately supplying the source gas and the reaction gas, the thin film is doped with the carbon element using a carbon-containing gas separate from the source gas and the reaction gas. This causes a concentration gradient of gas in the reaction tube in a vertical direction, which results in a variation in composition ratio of thin films between wafers. In other words, it is difficult to form a thin film having a desired composition ratio for each wafer in the reaction tube.
There are known a method of changing a processing pressure when a source gas is supplied or a method of supplying a source gas while stopping a process of exhausting a processing atmosphere, in the ALD process. In addition, there is known a method of repeating, a plurality of times, a step of supplying an ozone (O3) gas and a step of exhausting a processing atmosphere. However, such methods do not describe the aforementioned problems and related techniques.